1. Field of the Invention
This invention relates generally to a system and method for controlling a torque converter clutch and/or transmission gear shifts in a vehicle and, more particularly, to a system and method for controlling a torque converter clutch and/or transmission gear shifts in a vehicle that includes responding to the rate of change of the accelerator pedal position.
2. Discussion of the Related Art
Internal combustion engine vehicles that employ automatic transmissions typically include a torque converter positioned between the engine and the transmission of the vehicle. A torque converter is a fluid coupling device typically including an impeller coupled to an output shaft of the engine and a turbine coupled to the input shaft of the transmission. The torque converter uses hydraulic fluid to transfer rotational energy from the impeller to the turbine. Thus, the torque converter can disengage the engine crank shaft from the transmission input shaft during vehicle idling conditions to enable the vehicle to stop and/or to shift gears.
The rotational speed of the impeller relative to the turbine in the torque converter is typically different so that there is a converter slip therebetween. Because large slips between the engine output and the transmission input significantly affect the fuel economy of the vehicle, some vehicles employ a torque converter clutch (TCC) for controlling or reducing the slip between the engine and the transmission. The TCC can also mechanically lock the impeller at the output of the engine to the turbine at the input of the transmission so that the engine and transmission rotate at the same speed. Locking the impeller to the turbine is generally only used in limited circumstances because of various implications.
Thus, a TCC generally has three modes. A fully locked mode as just described, a fully released mode and a controlled slip mode. When the TCC is fully released, the slip between the impeller and the turbine of the torque converter is only controlled by the hydraulic fluid therebetween. In the slip mode, the TCC is controlled by the pressure of hydraulic fluid in the torque converter so that the slip between the torque converter impeller and the turbine can be set so that is does not exceed a predetermined slip.
A conventional gearshift schedule for automatic transmissions is implemented in the form of shift maps. A shift map is a group of table look-up functions that define gearshift points based on vehicle speed and throttle opening. Consequently, each gearshift point defined using conventional gearshift scheduling is a function of vehicle speed and throttle opening, and does not compensate for varying vehicle conditions, such as varying vehicle loads, or road conditions, such as road grade and curvature. However, it is desirable to dynamically generate gearshift points that use a combination of inputs that compensate for varying vehicle and road conditions to achieve optimal fuel economy, performance and drivability.
Typically, the schedules for torque converter clutch apply and release and transmission gear shifts are defined by accelerator pedal position and vehicle speed in automatic transmissions. When the vehicle driver increases the accelerator pedal to a larger angle, the transmission generally responds by opening the TCC and maybe down-shifting. However, there are times when a vehicle driver steadily pushes the accelerator pedal down to compensate for road load, road gradient, etc. to provide a constant vehicle speed. There are also times when the vehicle driver wants to pass another vehicle by providing a fast pedal position increase. The transmission control sometimes does not directly follow the driver's intention, and thus, provides unwanted TCC releases and/or down-shifts or up-shifts in some of these situations. For example, when the vehicle driver is steadily increasing the accelerator pedal position to maintain a constant vehicle speed when going uphill, it may not be desirable to provide a transmission gear down-shift that may occur because of the pedal position. Also, when a vehicle driver is passing another vehicle and makes a quick accelerator pedal down change, it may be desirable to provide the transmission gear down-shift more quickly that may otherwise be provided by the pedal position and the vehicle speed.